1. Field of the Invention
The present invention relates to a stencil printer, and more particularly, to a stencil sheet discharge device for discharging a used stencil sheet from a printing drum of a rotary type stencil printer.
2. Description of the Prior Art
A rotary stencil printer having a cylindrical printing drum is known. In such a rotary stencil printer, a stencil sheet perforated according to an image (called "master") is mounted over an outer circumference of a printing drum having an ink permeable wall structure with its leading end held by a clamp means, and print sheets are transferred one by one through a nip region between the printing drum and a back press roller, so that the print sheets are applied with a printed image by ink supplied to the inside of the printing drum and squeezed out through the perforated portions of the master.
In order to discharge a used master from the printing drum after the end of printing, a stencil sheet discharge device was proposed by the same assignee as that of the present application as described in Japanese Patent Laid-open Publication 7-32716. This stencil sheet discharge device comprises a pair of stencil sheet discharge rollers positioned close to the printing drum and is adapted to draw out the master from the printing drum, starting from its leading end, according to a rotation of the printing drum, and to transfer the master to a stencil sheet disposal spot.
The stencil sheet is generally a multi-layered sheet of a synthetic resin film and a perforated sheet such as Japanese paper, and is therefore apt to curl to the side of the resin film due to a difference in the thermal expansion coefficient between the two sheet materials. Since the master is mounted around the printing drum with its resin film side being positioned outside, with its leading end being fastened to the printing drum by a damp means, while its trailing end being left free, there is a tendency that the trailing end of the master lifts up from the circumferential surface of the printing drum according to such a curling of the master (called "upcurl" hereinbelow). Further, when the printing drum is rotating, the trailing end of the master not fastened to the circumferential surface of the printing drum is apt to lift up therefrom also according to a centrifugal force applied thereto. If the trailing end of the master touches the stencil sheet discharge rollers at its resin film surface due to the above-mentioned upcurl and/or the lifting up by the centrifugal force, the resin film surface is contaminated with the ink attached on the surface of the stencil sheet discharge roller, and there occurs an ink contamination of the back press, as it contacts with the resin film surface of the master. The ink contamination of the back press roller according to such a process proceeds with the repetition of the printing process, possibly causing a heavy ink contamination of the printer.